1. Field of the Invention
The present invention relates to an improvement of an axial air-gap vibration motor comprising an eccentric rotor used as silent call means for a mobile communications device.
2. Description of the Prior Art
The Applicant has proposed in Japanese patent 2137724 (U.S. Pat. No. 5,036,239) a flat coreless vibration motor wherein three air-core armature coils are disposed eccentrically on one side, thereby causing a rotor contained therein to be eccentric, and no output shaft is provided.
Because much of the effective conductor length of the armature coils is used, this motor is comparatively efficient; with input of 3V, 8 mA of consumable power can easily be obtained. In addition, because there are no output shaft or eccentric weight and thus the motor is not subject to the corresponding design restrictions, it can be used in many ways, and, further, there is no danger during rotation; for these and other reasons the motor was well received by the market. On the other hand, because the motor has three air-core windings on one side, the coils necessarily have to be made small. For this reason, when such a motor is miniaturized, it is difficult to position the effective conductor portion of the coils at the pitch angle of magnetization for a magnet, which is a reference electrical angle.
Recently, as disclosed in Japanese Laid-open Patent Application 2001-211624, motors have been proposed such that three air-core armature coils of large, middle and small size are caused to overlap in the radial direction at the same position, so that the spatial phases of the coils are equal.
However, with such a configuration, unless the neutral zone of the magnet used is made large, there will be a portion generating reverse torque. Therefore, because a motor thus configured will have a magnet with a small effective cross-section area, its efficiency is not good.
In addition, as disclosed in Japanese Laid-open Patent Application 2001-190052, there is a motor wherein two air-core coils of large and middle size are caused to overlap in the radial direction at the same position, so that the spatial phases are equal, and in place of a small coil a resistor is inserted. However, because a resistor that makes no contribution to torque is inserted, efficiency is degraded.
Further, with a motor comprising an internal eccentric rotor where three armature coils are disposed on one side, as described above, the more such a motor is miniaturized, the smaller the intervals between the armature windings, and it becomes difficult to connect the terminal ends thereof to a rectifier in a manner that the armature windings are not damaged. In addition, because the armature coils must be made smaller than the pitch angle of magnetization of the magnet, which is a reference electrical angle, greater improvements in efficiency are desired.